Jan 02, 2016

INTRODUCTION TOSURVIVAL ANALYSIS

What is Survival Analysis?Survival Analysis is referred to statistical methods for analyzing survival data

Survival data could be derived from laboratory studies of animals or from clinical and epidemiologic studies

Survival data could relate to outcomes for studying acute or chronic diseases

What is Survival Time?Survival time refers to a variable which measures the time from a particular starting time (e.g., time initiated the treatment) to a particular endpoint of interest (e.g., attaining certain functional abilities) It is important to note that for some subjects in the study a complete survival time may not be available due to censoring

Censored DataSome patients may still be alive or in remission at the end of the study period

The exact survival times of these subjects are unknown

These are called censored observation or censored times and can also occur when individuals are lost to follow-up after a period of study

Random Right Censoring

Suppose 4 patients with acute leukemia enter a clinical study for three years

Remission times of the four patients are recorded as 10, 15+, 35 and 40 months

15+ indicate that for one patient the remission time is greater than 15 months but the actual value is unknown

Important Areas of ApplicationClinical Trials (e.g., Recovery Time after heart surgery)

Longitudinal or Cohort Studies (e.g., Time to observing the event of interest)

Life Insurance (e.g., Time to file a claim)

Quality Control & Reliability in Manufacturing (e.g., The amount of force needed to damage a part such that it is not useable)

Survival Function or CurveLet T denote the survival time

S(t) = P(surviving longer than time t )= P(T > t)The function S(t) is also known as the cumulative survival function. 0 S( t ) 1

(t)=number of patients surviving longer than ttotal number of patients in the study

E.g: Four patients survival time are 10, 20, 35 and 40 months. Estimate the survival function.

Example: Four patients survival data are 10, 15+, 35 and 40 months. Estimate the survival function

In 1958, Product-Limit (P-L) method was introduced by Kaplan and Meier (K-M)As you move from left to right in estimation of the survival curve first assign equal weights to each observation. Do not jump at the censored observationsRedistribute equally the pre-assigned weight to the censored observations to all observations to the right of each censored observation

Median survival is a point of time when S(t) is 0.5Mean is equal to the area under the survival curve

A few critical features of P-L or K-M EstimatorThe PL method assumes that censoring is independent of the survival times

K-M estimates are limited to the time interval in which the observations fall

If the largest observation is uncensored, the PL estimate at that time equals zero

Comparison Of Two Survival Curves Let S1(t) and S2(t) be the survival functions of the two groups. The null hypothesis is H0: S1(t) =S2(t), for all t > 0

The alternative hypothesis is:H1: S1(t) S2(t), for some t > 0

The Logrank Test SPSS, SAS, S-Plus and many other statistical software packages have the capability of analyzing survival dataLogrank Test can be used to compare two survival curvesA p-value of less than 0.05 based on the Logrank test indicate a difference between the two survival curves

EXAMPLESurvival time of 30 patients with Acute Myeloid Leukemia (AML)

Two possible prognostic factors Age = 1 if Age of the patient 50 Age = 0 if Age of the patient < 50 Cellularity = 1 if cellularity of marrow clot section is 100% Cellularity = 0 otherwise

Format of the DATA Survival Times and Data of Two Possible Prognostic Factors of 30 AML Patients

* Censored = 1 if Lost to follow-up Censored = 0 if Data is Complete

Comparing the survival curves by Age Groups using Logrank Test

Comparing the survival curves by Cellularity using Logrank Test

Hazard FunctionThe hazard function h(t) of survival time T gives the conditional failure rate

The hazard function is also known as the instantaneous failure rate, force of mortality, and age-specific failure rate

The hazard function gives the risk of failure per unit time during the aging process

Multivariate Analysis: (CPHM)Cox's Proportional Hazards ModelCPHM is a technique for investigating the relationship between survival time and independent variables

A PHM possesses the property that different individuals have hazard functions that are proportional to one another

Comparing the survival curves by Age Groups after Adjusting Cellularity using CPHM

Comparing the survival curves by Cellularity Groups after Adjusting Age using CPHM

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